Add delete_values() and delete_undef_values() functions

Hi, 
`delete_values()` complements `delete()` for hashes - deletion is based on hash values, not keys,
`delete_undef_values()`  may be used to filter-out "unset" parameters from hash. It then plays nicely with `ensure_resource()` or `create_resources()` - see motivating example in README.markdown. I leave this pull request if you consider this function useful enough. Tell me if i've reinvented the wheel.

Regards.